The invention relates to a method and apparatus for selectively eliminating the oxides of nitrogen in the exhaust gases of furnace systems by introducing chemical compounds into the exhaust gases emerging from the furnace system for reaction with the oxides of nitrogen and by then passing the exhaust gases through a catalytic converter in order to accelerate and/or initiate the reaction, as well as to an apparatus for performing the method.
To selectively reduce the oxides of nitrogen, it is already known to meter out ammonia (NH.sub.3) in vapor form, in a mixture with air and dissolved in water either with or without pressure, into the flow of exhaust gas emerging from the furnace system. By means of a blending section having appropriate fixtures, the object is to develop a uniform distribution of ammonia and a uniform temperature distribution inside the flue gas conduits attached thereto. The mixture is then passed through a catalytic converter connected to the inlet side of a rotary regenerative heat exchanger intended for transmitting the heat of the exhaust gas to the combustion air that is supplied to the furnace. The catalyst is embodied as a fixed-bed reactor, the flow of which is preferably directed vertically downward. A honeycomb structure with a vanadium compound as its catalytically active substance is contained in the reactor. The pressure loss of the fixed-bed reactor is taken into account when dimensioning the flue gas blower. The vertically downward flow in the reactor is intended to counteract solid contaminant deposits in the reactor, or to keep such deposits within limits. The deposits that do occur are intermittently removed by blowing, using compressed air or steam. Depending on the boiler, the mode of operation, and the composition of the coal that is used for fuel, the catalytic converters inside the reactor have a service life of over two years.
Based on the recognition that the conversion of the oxides of nitrogen, which occur in proportions of approximately 95% nitrogen monoxide and approximately 5% nitrogen dioxide, into molecular nitrogen and water does not depend solely upon the ratio of the quantity of ammonia that is introduced to the content of nitrogen oxide in the exhaust gases and upon the temperature level of the exhaust gases, but instead is substantially affected by the efficiency of the catalytic converter as well, it is the object of the invention to devise a method and an apparatus for selectively eliminating the oxides of nitrogen in furnace exhaust gases using a catalytic converter, in which the catalytic properties of the catalyst elements used are maintained over relatively long periods of operation.